1. Field of the Invention
The present invention relates to a low distortion ratio hydrophone of piezoelectric type for example.
2. Description of the Prior Art
The prior art in the field of piezoelectric hydrophones is notably illustrated in French Pat. Nos. 1,556,971; 2,122,675; 2,733,831 and 2,748,183, U.S. Pat. Nos. 3,970,878; 4,336,639; 4,926,397; 5,541,894; 5,815,466 and 5,677,894, and in French Patent Application 99/05,289.
It is well-known to form hydrophones by assembling elements sensitive to pressure variations, consisting each of a disc made of a piezoelectric ceramic associated with a pair of electrodes arranged on either side. Each disc is attached onto a flexible support such as a diaphragm-4 one of the faces thereof being exposed to the pressure variations to be measured.
The flexible support is for example a diaphragm supported by a rigid housing or by the flexible central part of a cup whose periphery is reinforced and rigid, resting against an identical cup, itself carrying one or more sensitive elements, the two cups defining a housing. The electrodes of the two sensitive elements are electrically interconnected so as to provide compensation for the parasitic effects due to accelerations.
When the outside static pressure increases, the two plates bend until they rest against each other. The space between them is so selected that their maximum deformation, when they are pressed against each other, remains within elastic deformation limits. The pickup is thus protected against accidental overpressures. The housing thus formed can be coated with a protective layer made of a material transparent to acoustic waves.
The sensitive elements can be externally fastened to the housing and covered with a protecting coating (such as a varnished araldite layer) so as to maintain a sufficient electric insulation between the electrodes. The sensitivity of this type of hydrophone is good and does not vary much (less than 10% for a static pressure of 10 Mpa for example) with the hydrostatic pressure.
According to another well-known layout, the sensitive elements are fastened to the inner faces of the cups and therefore inside the housing, which provides good protection against the outside medium but they may thus be damaged through crushing when the hydrostatic pressure increase presses them against each other by bending the plates. The sensitivity of hydrophones with sensitive elements inside the housing is lower and it greatly decreases when the hydrostatic pressure increases, a phenomenon that can be attributed to their being fastened to a face that becomes convex.
A common drawback of symmetrically connected hydrophones with a single pair of sensitive elements, both inside or outside the housing, is the relatively great variation of their capacitance with the hydrostatic pressure. Capacitance increases with the pressure when the sensitive elements are inside, whereas capacitance decreases when the sensitive elements are outside.
It is also well-known to combine the previous two types in a hydrophone with two diaphragms defining a closed housing, each one carrying a pair of sensitive elements, one outside the housing, the other inside, the two pairs being electrically interconnected in parallel so as to compensate for certain drawbacks inherent in the two layout modes mentioned above, but this solution is relatively expensive.
French Patent Application 99/05,289 mentioned above describes a hydrophone comprising a closed housing defined by two cups with a flexible central part and at least two piezoelectric sensitive elements of different sizes associated each with electrodes and electrically interconnected, these two sensitive elements being respectively fastened to the diaphragms, one outside the housing, the other inside the housing. Such a hydrophone with a single pair of sensitive elements combines the advantages of the prior hydrophones. The inter-electrode capacitance is substantially constant within a wide pressure range and the sensitivity is of the same order of magnitude as that obtained with a symmetrical connection outside the housing.
The aforementioned U.S. Pat. No. 5,677,894 describes a piezoelectric type hydrophone comprising an elongate housing with opposite flexible diaphragms each carrying externally a piezoelectric crystal made of several parts separated from one another: a central part and two lateral parts arranged symmetrically and electrically connected to the central part. A rigid element (a rod) is inserted between the two diaphragms so that application of a pressure causes the central parts of the sensitive crystals to become convex and their lateral parts to become concave. The crystals are so sized that, by combination of the electric charges due to these opposite effects, distortion is reduced.
The hydrophone according to the invention comprises a housing including at least one diaphragm and at least one pair of sensitive elements (of piezoelectric, optical fiber type, etc.) in contact with the housing, which generate signals indicating the generated stresses of at least one diaphragm of the housing, in response to pressure variations in the outside medium.
The first sensitive element of each pair is placed in contact with a first diaphragm portion undergoing essentially linear deformations, and in that the second sensitive element of each pair is in contact with at least a second diaphragm portion undergoing essentially less linear deformations, the respective areas of the first and second portions are selected and the signals produced by the two sensitive elements of each pair are combined so as to substantially compensate for the distortion affecting the resulting signals produced in operation by the hydrophone.
The housing of the hydrophone comprises for example at least one cup with a flexible central part and a more rigid external part, the first sensitive element of each pair generates signals indicating the essentially linear deformations affecting a flexible central part of a diaphragm, and the second sensitive element of each pair generates signals indicating the essentially less linear deformations affecting a more rigid external portion of the diaphragm.
In the case for example where the sensitive elements of each pair are of a piezoelectric type with each two opposite faces and electrodes associated with these opposite faces, at least a first electrode of the second sensitive element of the pair covers only a portion of the face of the second sensitive element against which the second sensitive element presses (an external portion for example), the second electrode of the second sensitive element covering substantially the total face against which the second sensitive element presses or only an external portion of each of the faces thereof.
According to an embodiment, the housing includes a flexible diaphragm and the two sensitive elements of the same pair are respectively fastened to the opposite faces of the diaphragm.
According to another embodiment, the housing includes two flexible diaphragms, the two sensitive elements of the same pair are respectively fastened to either corresponding faces (both external or internal), or to different faces of these two flexible diaphragms.
According to another embodiment, the hydrophone comprises two pairs of sensitive elements respectively fastened to the two flexible diaphragms, the corresponding sensitive elements of the two pairs of sensitive elements are respectively in contact with either the identical faces or with the opposite faces of the two diaphragms.
According to another embodiment, the two sensitive elements of at least one pair of sensitive elements are different in size and, if they are piezoelectric sensitive elements, at least one of the electrodes associated with the larger sensitive element covers only an external portion of the corresponding face.
The portions of the faces covered by the electrodes on each sensitive element of a pair are preferably dimensioned so as to compensate for the distortion.
According to another embodiment, the housing comprises at least one circular flexible diaphragm, the sensitive elements of each pair are discs made sensitive, one on a central portion, the other on a peripheral part.
As the case may be, the sensitive elements of each pair are either directly interconnected or connected by means of an adapter element.
Manufacture of the hydrophone defined above is simplified. In the case for example where piezoelectric type sensitive elements are used, it is not necessary to cut up crystals to form each peripheral compensation sensitive element. The crystal is entire but only the peripheral portions selectively metallized on both sides contribute to creating in operation electric charges taking part in the desired distortion compensation and reduction. When the discs of each pair are fastened to the opposite faces of the same flexible diaphragm, they rigidify the diaphragm. This is useful for limiting its curvature towards the inside of the housing when the outside hydrostatic pressure increases as a result of an immersion depth variation.
It is experimentally verified that the residual distortion affecting the signals produced through this layout, notably the distortion due to the second harmonic of the signals, is greatly reduced and becomes practically zero if the dimensions of the (or of each of the two) peripheral electrode(s) of each first sensitive element are optimized.